SeanBiggerstaff.com had this to say regarding the circulating rumours that he’s not slated for PoA:

We have received a number of inquires as to Sean and his position with the upcoming Prisoner of Azkaban movie. There have been rumours circulating that Sean will not be returning as Oliver Wood. Please understand that rumours are exactly that and are not based on facts.
Unfortunetely we are unable to comment on Sean’s talks regarding the filming and/or script.

Precisely – rumours. And as some of us TLC editors have discussed offline, we sincerely hope this particular rumour turns out to be baseless. Oliver is extremely important to PoA Quidditch – and that means Sean is extremely important to PoA movie Quidditch – and we just … hope the movie does not drop the Quidditch (though doing justice to the main Sirius/etc. story line is far more important, we realize.